1. Technical Field
The present disclosure relates to a vertical power component capable of withstanding a high voltage (for example, greater than 500 V), and more specifically relates to the peripheral structure of such a component.
2. Discussion of the Related Art
FIG. 1 is a cross-section view of a vertical power component, illustrating a way of forming the periphery of a component in so-called “planar” technology to avoid the occurrence of breakdowns at the component edges.
The component shown in this example is a triac comprising a lightly-doped N-type silicon substrate 1 (N−), currently with a doping ranging from 1014 to 1015 atoms/cm3, having its upper and lower surfaces comprising P-type doped wells 3 and 5 (P) extending almost over the entire surface of the component, except at the periphery thereof. Upper well 3 contains a heavily-doped N-type region 4 (N+), and lower well 5 contains a heavily-doped N-type region 6 (N+) in an area substantially complementary to that occupied by region 4. Upper well 3 further contains a small heavily-doped N-type region 8 (N+). On the lower surface side of the component, an electrode A2 coats well 5 and region 6, and an insulating layer 11, for example, made of silicon oxide, coats the peripheral portion of the lower surface which is not covered with electrode A2. On the upper surface side of the component, an electrode A1 coats region 4 and a portion of well 3, a gate electrode G coats region 8 and a portion of well 3, and an insulating layer 9, for example, made of silicon oxide, coats the portions of the upper surface which are not covered by electrodes A1 and G. Whatever the biasing between electrodes A2 and A1, if a gate control signal is provided, the component becomes conductive. The conduction occurs from electrode A2 to electrode A1 through a vertical thyristor comprising regions 5, 1, 3, and 4, or from electrode A1 to electrode A2 through a vertical thyristor comprising regions 3, 1, 5, and 6. The thickness and the doping level of substrate 1 are calculated so that the triac, in the off state, can withstand high voltages, for example, voltages ranging between 600 and 800 volts.
To avoid that breakdowns occur at the component edges, a distance should be provided between the limit of P-type wells 3 and 5 and the component edge. In this example, to the upper and lower peripheries of the substrate are further surrounded with a heavily-doped N-type channel stop ring (N+), respectively 13 and 14, which helps avoiding breakdowns at the level of the component edges.
A disadvantage of such a structure is due to the fact that the lateral surfaces of substrate 1 are not insulated. Thus, when lower electrode A2 of the component is soldered to a contact area 15 of an electronic device (metal plate of a radiator, printed circuit board, package, etc.), it is necessary to ascertain that lateral wickings 16 do not electrically connect electrode A2 to substrate 1, which would short-circuit the PN− junction formed between well 5 and substrate 1. In practice, as illustrated in FIG. 1, one arranges, between electrode A2 and area 15, an intercalary conductor 17, or pedestal, of smaller surface area (in top view) than the component, to raise the component so that excess solders 16 remain contained on its lower surface side and do not creep up along its lateral walls. The provision of pedestal 17 further enables to decrease the intensity of the electric field to which insulating layer 11 is submitted, due to the strong potential difference between substrate 1 and channel stop ring 14 on the one hand, and metal contact area 15 on the other hand. A disadvantage is that the provision of pedestal 17 significantly complicates the component assembly.
FIG. 2 is a cross-section view of a vertical power component corresponding to another conventional component periphery structure in planar technology. FIG. 2 shows a triac comprising the same elements as the triac of FIG. 1. The component of FIG. 2 differs from that of FIG. 1 by the structure of its periphery. The periphery of the component of FIG. 2 is surrounded with a P-type diffused wall 21 (P) formed from the lower and upper surfaces of the substrate. On the lower surface side of the component, well 5 extends laterally all the way to diffused wall 21. On the upper surface side of the component, well 3 stops before diffused wall 21, and a heavily-doped N-type channel stop ring 23 (N+) is arranged between well 3 and wall 21 and surrounds well 3. Lower electrode A2 coats the entire lower surface of the component. On the upper surface side, electrode A1 coats region 4 and a portion of well 3, gate electrode G coats region 8 and a portion of well 3, a ring-shaped electrode 24 coats channel stop ring 23, and an insulating layer 9 coats the portions of the upper surface which are not covered with the electrodes.
To avoid that breakdowns occur at the component edges, and due to the presence of channel stop ring 23, a distance should be provided between the limit of P-type well 3 and diffused wall 21.
The structure of FIG. 2 has the advantage, as compared with the structure of FIG. 1, that the lateral surfaces of substrate 1 are insulated by diffused wall 21, which prevents any risk of short-circuit between electrode A2 and substrate 1 due to possible lateral wickings on assembly of the component. It is thus not necessary to provide an intercalary conductive element between electrode A2 and contact area 15. Further, the breakdown area is transferred to the upper surface side.
However, a disadvantage of the structure of FIG. 2 is that guard distance e2 between the edge of the component and the beginning of electrode A1 or G, respectively (limit of the actual active portion of the component), is larger than guard distance e1 between the edge of the component and the beginning of electrode A1 or G, respectively, in the structure of FIG. 1. As an example, to obtain a breakdown voltage greater than 800 volts, a guard distance e2 on the order of 400 μm must be provided in the structure of FIG. 2, to be compared with a guard distance el on the order of 200 μm in the structure of FIG. 1. This thus decreases the available surface area for the electrodes of the component of FIG. 2; or, for given electrode surface area values, this increases the component surface area, and thus its cost.
Another disadvantage of the structure of FIG. 2 is that the forming of lateral wall 21 requires a very long (and thus expensive) step of diffusion of dopant elements from the upper and lower surfaces of the substrate (typically on the order of 250 hours for a substrate having a thickness ranging from 200 to 300 μm and a boron doping).